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i still seam to be having the same problem? i made the holes 3.2mm but it is not till i reduce the cutter to 2.6 that it seams to want to include them? any ideas is there another setting i have got wrong? I have attached the file hopefully it shows the setting. many thanks

i still seam to be having the same problem? i made the holes 3.2mm but it is not till i reduce the cutter to 2.6 that it seams to want to include them? any ideas is there another setting i have got wrong? I have attached the file hopefully it shows the setting. many thanks

Because you need to use a drill operation for the holes.

Spelling mistakes are not intentional, I only seem to see them some time after I've posted

I have been routing all my holes this way for years. I wonder if Charlie is putting his cutter paths on the circle centre with no offset to the inside of circle? To my mind the problem will be in that direction. G.

This is the bit I don't understand, why make the hole bigger if a 3mm hole is required ? Sometimes this will be fine but I'm sure there are more instances where the measurement has to be right.

The way I see it is If you want the hole to be 3mm and you also want to profile it out then you have to use a smaller end mill say 2.5mm BUT if you have a job like Charlie is doing and he is wanting to use a 3mm end mill to do the total job and the 3mm hole size does not have to be 3mm then make it say 3.01 and use the same cutter to save changing bits (his holes are only for mounting holes anyway).
I did a very similar job for a quadcoptor all with one size bit and did the same just made the 3mm holes to 3.01.
.
Or like has already been suggested peck drill it

That is why I suggested making the hole diameter slightly bigger and routing it out - this has worked for me for years!!! Of course if you have a drill cycle option that works fine as well. G.

So what if there are say 166 holes.? Do you change ever piece of geometry.? how longs that going to take.! (Answer In this case would actually be to tell Cam tool is 2.99mm but hole will still be wrong size)
What if the holes are for dowl pins and need to be exactly 3mm.? In this case a 3mm endmill won't work so again more time wasted swapping out endmills.

Maybe I'm being a bit Pedantic here but Better to get into a habbit of using the easiest and less disruptive option which in this case would have been a Peck drill than changing geometry or using fudges like under sizing tools in Cam.